The present invention relates to electrically weldable bushings or connectors for joining plastic pipe, hose, or the like.
Fluid supply networks and conduits for other uses are frequently produced by using line or pipe elements of thermoplastic material. The problem is to assemble the line or pipe elements, including pipe sections, elbow joints, molded sections, control or valve elements and other workpieces so that they are both drip and pressure-tight at the connections. Cylindrical bushings, i.e. sleeve-shaped bushings, of thermoplastic material are generally used to join the ends or socket portions of such line elements and together with the line elements are partly melted and welded to each other. The bushing can be an integral part of the line element or it can be formed as a separate part fitting over the end of the bushing. To effect the partial melting and welding of the parts, a resistance wire is arranged in helical turns, just below the surface of the inner wall of the bushing. By supplying electrical current to this resistance wire, a predetermined amount of heat is generated, sufficient to partially melt and join the bushing body and the ends of the line elements.
In the patents to STURM, U.S. Nos. 4,224,505; 4,117,311; and, 3,943,334; and LIPPERA, U.S. No. 4,176,274, resistance wire having a high coefficient of expansion was used.
To obtain the proper amount of heat necessary for welding, known electric power supply system have been employed, furnishing electrical current proportional to the required amount of heat necessary for the embedded filamentary resistance wire to itself heat and melt the thermoplastic material. Various power supply systems are known, which not only supply the necessary current, but which maintain a given welding time, regulated to a constant voltage or constant current. Apparatus is also known having temperature measuring sensors placed on the bushing itself, to determine the heat levels. Even systems wherein the ambient temperature of the bushing is taken into account in the regulation of the welding time are known.
While the bushing, and its embedded filament resistance wire, capable of producing an electrical weld, seems to be relatively simple, care must be taken that when the filament wire is inserted in the bushing, the individual turns are spaced from each other by a sufficiently large plastic mass to positively avoid interturn short-circuiting. On the other hand, in order to obtain a good welding of the parts to each other, the bushing must be provided with a shrinkage reserve allowing it to contract under the welding operation, so that a tight fit between the bushing and the sections of the line elements to be joined, is ensured. However, while the bushing body diminishes in size during welding, the filament resistance wire, which is heated during welding, expands opposite to the shrinking movement of the bushing. These two opposite forces tend to cause breaking of the individual turns of the resistance wire and thus create breaks and/or short-circuits, so that the thermoplastic material itself, forming the bushing, may be damaged or even ignited. Furthermore, the elongation of the resistance wire, under heat, causes the turns to move away from the welding zone, that is, radially away from the welding interface. The distance the turns move can become so great that complete melting of the inner wall in the welding zone may be seriously diminished and jeopardized. Such phenomena can be observed, particularly in bushings having large diameters, for example, over 200 mm. in diameter. The thermal expansion of the resistance wire undoubtedly plays a significant role in this enlargement, since the total length of the wire turns can be considerable, for example, 10 m. or more.
Various measures have been taken to avoid the above-described difficulties, particularly in welding of bushings with large diameters. For example, the total length of a filament wire can be reduced by inserting the filament into the bushing in several independent strands or threads; even four sections are known. The total expansion of the filament wire is thus reduced, so that the risk in interturn short-circuiting is likewise reduced, but here, the electric circuit for the supply to these wires, is increased in complexity.
It has furthermore been known to use a varnish or tape insulated wire. Though the thermal expansion of the filament wire is not reduced by such insulation, the insulation tends to prevent interturn short-circuiting, should such turns in fact contact each other. The insulation of commercial filament wires, however, is so thin that it is normally not sufficient to prevent short-circuits, and therefore, insulated wire has to be custom-made.
In addition, there is a risk that varnish insulation will be charred at high filament wire temperatures, giving off, as a result, solvent vapors. These released gases lead to cavitation in the interface welding zone. By contract, no gases are usually released in a tape insulated filament wire, although it is well known that tape insulation is rather expensive and fificult to handle.
It is the object of the present invention to overcome the foregoing difficulties and to provide a weldable bushing of the type described above, in which interturn short-circuits are avoided, and the filament resistance wire turns remain stationary in their positions and prevent disturbances, particularly in bushings having large diameters.